Cable Size Calculator — South Africa
Find the correct cable size for any SA circuit. Checks current carrying capacity and voltage drop against SANS 10142 requirements.
Quick answer: A 3kW geyser on a 230V single-phase supply draws approximately 13A, requiring a minimum 2.5mm² copper cable for short runs. SANS 10142 also requires checking voltage drop — for runs over 20m, a 4mm² cable may be needed to keep voltage drop within the 5% (11.5V) limit.
⚡ Quick fill — tap a common circuit:
⚠️ For planning purposes only. All electrical work in SA must be carried out by a registered electrician under SANS 10142. A Certificate of Compliance (CoC) is required.
How to Use This Calculator
Tap a common circuit above to quick-fill, or enter your own values. Choose Watts (from the appliance nameplate) or Amps (from your DB board), select 230V single-phase or 400V three-phase, then enter the one-way cable run length from the DB board to the load.
Select the installation method — clipped/surface-mounted cable runs cooler and carries more current than the same cable in conduit. The calculator checks both SANS 10142 requirements (current capacity AND voltage drop) and shows the minimum compliant cable size.
Cable Sizing in South Africa — SANS 10142 Explained
Selecting the correct cable size is one of the most critical steps in any electrical installation. An undersized cable overheats, degrades insulation, creates a fire risk, and will fail a Certificate of Compliance (CoC) inspection. Oversizing wastes money unnecessarily. SANS 10142 — South Africa's wiring code — specifies exactly how cable sizing must be done.
There are two separate tests a cable must pass under SANS 10142, and both must be satisfied independently. This calculator checks both automatically.
The Two SANS 10142 Cable Sizing Checks
Check 1 — Current Carrying Capacity
Every cable has a maximum continuous current it can carry safely without the insulation overheating. For standard copper PVC-insulated cables at 70°C operating temperature, these are the rated values used in South Africa:
| Cable Size | Clipped Direct (A) | In Conduit (A) | Volt Drop (mV/A/m) | Typical Use |
|---|---|---|---|---|
| 1.5mm² | 17.5A | 14A | 28.0 | Lighting circuits |
| 2.5mm² | 25A | 20A | 17.0 | Plugs, geysers (short runs) |
| 4mm² | 32A | 26A | 11.0 | Geysers, cooktops (short runs) |
| 6mm² | 41A | 34A | 7.1 | Stoves, pool pumps |
| 10mm² | 57A | 46A | 4.2 | Large stoves, sub-mains |
| 16mm² | 75A | 61A | 2.7 | Sub-distribution boards |
| 25mm² | 100A | 80A | 1.7 | Main DB feeds |
| 35mm² | 125A | 100A | 1.3 | Main supply cables |
Source: MicCom Cables SA / SANS 10142 aligned. Values for copper PVC-insulated cables, conductor operating temperature 70°C.
Check 2 — Voltage Drop
Even if a cable can carry the current safely, it must not drop too much voltage along its length. SANS 10142 sets the maximum voltage drop at 5% of the supply voltage for final circuits:
- 230V single phase: Maximum 11.5V drop
- 400V three phase: Maximum 20V drop
Current (A) = Power (W) ÷ Voltage (V) [single phase]
Current (A) = Power (W) ÷ (400 × 1.732) [three phase]
Voltage Drop (V) = mV/A/m × Current (A) × Length (m) ÷ 1000
VD % = Voltage Drop (V) ÷ Supply Voltage (V) × 100
SANS 10142 limit = 5% of supply voltage (11.5V / 20V)
Why Installation Method Matters
A cable installed in conduit or enclosed trunking cannot dissipate heat as efficiently as one clipped to a surface or run in open air. This means the same cable has a lower current rating in conduit than when surface-mounted. The SANS 10142 derating for conduit is significant — a 2.5mm² cable drops from 25A (clipped) to 20A (in conduit). Failing to apply this derating results in overloaded cables that overheat and degrade.
Common SA Circuit Examples
These are typical cable sizes used in South African residential installations. Always verify with a registered electrician, as actual installations depend on run length, grouping, and ambient temperature.
- Lighting circuits: 1.5mm² (up to 20m runs), 2.5mm² for longer runs
- 15A plug circuits: 2.5mm² (short runs), 4mm² (over 25m)
- Geyser 3kW: 2.5mm² (under 15m), 4mm² (15–30m)
- Electric stove 6–9kW: 6mm² minimum, 10mm² for longer runs
- Air conditioner 3–5kW: 2.5mm² (short runs), 4mm² (over 15m)
- DB board sub-mains: 10mm²–25mm² depending on total load
- Inverter / solar feed: 10mm²–25mm² depending on inverter size and run length
Ambient Temperature Derating — Critical in South African Conditions
The current-carrying capacity values in SANS 10142 assume an ambient temperature of 30°C. In South African conditions — roof spaces in summer, industrial environments, and direct-sun surface installations — ambient temperatures frequently exceed 40°C, which significantly reduces cable capacity. SANS 10142 specifies derating factors: at 40°C the derating factor is 0.91 (capacity is 91% of the table value), at 50°C it drops to 0.82, and at 60°C to 0.71. A 2.5mm² cable rated at 25A clipped direct at 30°C is effectively limited to 20A in a 50°C roof space — the same rating as a conduit installation. Always apply the ambient temperature derating factor for roof space installations, particularly for geyser circuits and solar panel DC cabling installed in enclosed roof voids.
Grouping Derating — When Cables Run Together
When multiple current-carrying cables are bundled or run in close contact, the combined heat from adjacent cables reduces each cable’s individual capacity. SANS 10142 specifies grouping derating factors: 2 cables together requires a factor of 0.80 (capacity is 80% of single-cable value), 3 cables require 0.70, and 5 or more cables require 0.57. This is particularly relevant in South African installations where multiple circuits are routed in a single conduit or cable tray between the DB board and distribution points. A 2.5mm² cable carrying 20A in a conduit is safely loaded when run alone, but may be overloaded when sharing the conduit with two other loaded circuits. Registered electricians are required to apply all applicable derating factors before specifying cable size.